The art of lithographic printing is based upon the immiscibility of oil and water, wherein an oily material or ink is preferentially retained by imaged areas and the water or fountain solution is preferentially retained by the non-imaged areas. When a suitably prepared surface is moistened with water and ink is applied, the background or non-imaged areas retain the water and repel the ink while the imaged areas accept the ink and repel the water. The ink is then transferred to the surface of a suitable substrate, such as cloth, paper or metal, thereby reproducing the image.
Very common lithographic printing plates include a metal or polymer support having thereon an imaging layer sensitive to visible or UV light. Both positive- and negative-working printing plates can be prepared in this fashion. Upon exposure to a patterned light image, and perhaps post-exposure heating, either imaged or non-imaged areas are removed using wet processing chemistries.
“Direct-write” imaging avoids the need for patterned light imaging and chemical processing. Direct-write using an infrared radiation laser is a thermally driven process and is more desirable because the laser heats only a small region at a time. Moreover, computer control allows for high resolution images to be generated at high speed since the images can be produced directly on the imaging member surface, pixel by pixel. The chemical processing steps are eliminated.
Examples of thermally sensitive printing plates are described in U.S. Pat. No. 5,372,915 (Haley et al.). They include an imaging layer comprising a mixture of dissolvable polymers and an infrared radiation absorbing compound. While these plates can be imaged using lasers and digital information, they still require wet processing using alkaline developer solutions.
It has been recognized that a lithographic printing plate could be created by ablating an IR absorbing layer. For example, Canadian 1,050,805 (Eames) discloses a dry planographic printing plate comprising an ink receptive substrate, an overlying silicone rubber layer, and an interposed layer comprised of laser energy absorbing particles (such as carbon particles) in a self-oxidizing binder (such as nitrocellulose). Such plates were exposed to focused near IR radiation with a Nd++YAG laser. The absorbing layer converted the infrared energy to heat thus partially loosening, vaporizing or ablating the absorber layer and the overlying silicone rubber. Similar plates are described in Research Disclosure 19201, 1980 as having vacuum-evaporated metal layers to absorb laser radiation in order to facilitate the removal of a silicone rubber overcoated layer. These plates were developed by wetting with hexane and rubbing.
While the noted printing plates used for digital, processless printing have a number of advantages over the more conventional photosensitive printing plates, there are a number of disadvantages with their use. The process of ablation creates debris and vaporized materials that must be collected. The laser power required for ablation can be considerably high, and the components of such printing plates may be expensive, difficult to coat, or unacceptable for resulting printing quality. Such plates generally require at least two coated layers on a support.
U.S. Pat. No. 3,650,743 (Hallman et al.) and U.S. Pat. No. 4,115,127 (Ikeda et) describe printing plates that do not require post-imaging processing. However, these imaging members are constructed of toxic materials such as arsenic that must be applied by vacuum deposition of mixed inorganic coating formulations.
Thermal or laser mass transfer is another method of preparing processless lithographic printing plates. Such methods are described for example in U.S. Pat. No. 5,460,918 (Ali et al.) wherein a hydrophobic image is transferred from a donor sheet to a microporous hydrophilic crosslinked silicated surface of the receiver sheet. U.S. Pat. No. 3,964,389 (Peterson) describes a process of laser transfer of an image from a donor material to a receiver material requiring a high temperature post-heating step.
Still another method of imaging that avoids chemical processing is the use of materials comprising microencapsulated hydrophobic materials as described for example in U.S. Pat. No. 5,569,573 (Takahashi et al.). Upon thermal imaging, the microcapsules rupture in an imagewise fashion to provide an ink-receptive image.
Thermally switchable polymers have been described for use as imaging materials in printing plates. By “switchable” is meant that the polymer is rendered from hydrophobic to relatively more hydrophilic or, conversely from hydrophilic to relatively more hydrophobic, upon exposure to heat. U.S. Pat. No. 4,034,183 (Uhlig) describes the use of high powered lasers to convert hydrophilic surface layers to hydrophobic surfaces. A similar process is described for converting polyamic acids into polyimides through a transparency mask in U.S. Pat. No. 4,081,572 (Pacansky). The use of high-powered lasers is undesirable in the industry because of their high electrical power requirements and because of their need for cooling and frequent maintenance.
U.S. Pat. No. 4,634,659 (Esumi et al.) describes imagewise irradiating hydrophobic polymer coatings to render exposed regions more hydrophilic in nature. While this concept was one of the early applications of converting surface characteristics in printing plates, it has the disadvantages of requiring long UV light exposure times (up to 60 minutes), and the plate's use is in a positive-working mode only.
U.S. Pat. No. 4,405,705 (Etoh et al.) and U.S. Pat. No. 4,548,893 (Lee et al.) describe amine-containing polymers for photosensitive materials used in non-thermal processes. Thermal processes using polyamic acids and vinyl polymers with pendant quaternary ammonium groups are described in U.S. Pat. No. 4,693,958 (Schwartz et al.). Such materials require aqueous processing after imaging. U.S. Pat. No. 5,512,418 (Ma) describes the use of polymers having cationic quaternary ammonium groups that are heat-sensitive. However, the materials described in this art require wet processing after imaging.
WO 92/09934 (Vogel et al.) describes photosensitive compositions containing a photoacid generator and a polymer with acid labile tetrahydropyranyl or activated ester groups. However, imaging of these compositions converts the imaged areas from hydrophobic to hydrophilic in nature.
EP 0 652 483A1 (Ellis et al.) describes direct-write lithographic printing plates imageable using IR lasers, and that do not require wet processing. These plates comprise an imaging layer that becomes more hydrophilic upon imagewise exposure to heat. This coating contains a polymer having pendant groups (such as t-alkyl carboxylates) that are capable of reacting under heat or acid to form more polar, hydrophilic groups. Imaging such compositions converts the imaged areas from hydrophobic to relatively more hydrophilic in nature, and thus requires imaging the background of the plate, which is generally a larger area.
U.S. Pat. No. 5,985,514 (Zheng et al.) describes printing plates containing heat-sensitive thiosulfate polymers that can be rendered hydrophobic upon imagewise application of thermal energy.
The graphic arts industry is seeking an alternative means for providing processless, direct-write, negative-working lithographic imaging members that can be imaged without ablation, or the other problems noted above, to provide high sensitivity, high imaging speed, long shelf life, and long press life.